The present invention disclosed herein relates to a lithium secondary battery positive electrode active material for improving output characteristics and a lithium secondary battery including the same.
As the development of techniques and demand for mobile devices are increasing, the demand of secondary batteries as an energy source has been rapidly growing. Among secondary batteries, a lithium secondary battery having a high energy density and voltage, a long cycle life span, and a low self-discharge rate, has been commercialized and widely used. Furthermore, as people are increasingly interest in the environment issues, studies for an electric vehicle, a hybrid electric vehicle, or the like, which may replace the vehicles, such as a gasoline vehicle, a diesel vehicle, or the like, using fossil fuel, one of major causes of air pollution, has been greatly conducted. Recently, studies into the use of a lithium secondary battery having a high energy density and discharge voltage as a power source of an electric vehicle, a hybrid electric vehicle, or the like, is actively ongoing and a lithium secondary battery is partially in a commercialization stage.
In particular, studies for developing a positive electrode material of a large capacity lithium secondary battery to be used for an electric vehicle are comprehensively carried out to replace currently used LICoO2.
In case of LiCoO2 that is the existing typical positive material, the LiCoO2 has reached its limit in an increase in an energy density and a practical use of output characteristics, and in particular, when LiCoO2 is used in a high energy density application field, its structure is denatured at a high charge state due to a structural instability and oxygen in the structure is discharged to cause an exothermic reaction with electrolyte in a battery to mainly cause a battery explosion. Thus, in order to improve the instability of LiCoO2, the use of a lithium-containing manganese oxide and lithium-containing nickel oxide (LiNiO2) such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, or the like, has been considered. Recently, a great deal of studies into the use of 3-component system layered oxides of Ni, Mn, and Co has been done.
An oxide expressed as chemical formula Li[Ni1/3Co1/3Mn1/3]O2, that is the most typical layered oxide among the 3-component system layered oxides, is changed from Ni2+ to Ni3+ or Ni4+ according to the depth of charge when charging is performed. However, unlike stable Ni2+, Ni3+ or Ni4+ (in particular Ni4+) loses lattice oxygen due to instability so as to be reduced to Ni2+, and the lattice oxygen reacts with electrolyte to change the surface qualities of an electrode or increase a charge transfer impedance of the surface of the electrode to reduce the capacity or degrade high rate capability.
In order to improve the limitation of the instability of the 3-component system layered oxide, technologies for mixing a metal oxide having an existing olivine structure with the 3-component system positive electrode active material have been known.
In particular, LiFePO4 positive electrode material having an olivine structure using Fe has been come into the spotlight because of stability of the crystal structure and low costs of the Fe. Thus, a mixture of the LiFePO4 and the 3-component system layered oxide having the above-described advantages has been proposed as a positive electrode active material having improved stability.
However, in a case of the positive electrode active material containing the mixture of the LiFePO4 having the olivine structure and the 3-component system layered oxide, the positive electrode active material has a higher electrical resistance even in a state of the same open circuit voltage (OCV) when compared that the same 3-component system layered oxide is applied by itself alone. Thus, the positive electrode active material containing the mixture of the LiFePO4 having the olivine structure and the 3-component system layered oxide has low output characteristics due to low conductivity in spite of the advantages of the low cost and high stability. Accordingly, a state of charge (SOC) area satisfying a required output is narrow, and thus an available SOC area may be limited.
To improve the limitation of the positive electrode material containing the mixture of the metal oxide having the olivine structure and the 3-component system metal oxide having the layered structure, there have been attempts to improve the mixed positive electrode material by adding a large amount of conductive material to reduce an electrical resistance. However, when the large amount of conductive material is added, the mixed positive electrode material has a high resistance as ever even though a ratio of the active material may be reduce to significantly reduce energy density. Accordingly, the limitations of the reduction of the output characteristics and the limited available SOC area remain as ever.
The low output characteristics are a problem to be solved for using the lithium secondary batteries as medium-large size secondary batteries for electric vehicles. Therefore, studies with respect to secondary batteries having widely available SOC area while maintaining a high output are urgently required.